Intertube transformer for vacuum-tube amplifiers



March 27, 1928; 1,664,239

M. C; BATSEL INTERTUBE TRANSFORMER FOR VACUUM TUBE AMPLIFIERS Filed March 5, 1921 N 3 D g. X

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WITNESSES: INVENTOR I J1 ATTORNEY Patented Mar. 27, 1928.

UNITED STATES MAX C. BATSEL, OF WILKINSBURG, PENNSYLVANIA.

INTER-TUBE TRANSFORMER FOR VACUUM-TUBE AMPLIFIERS.

Application filed March 5, 1921. Serial No. 449,611.

This invention relates to the coupling of transformers and more particularly to the coupling of transformers for connecting vacuum tubes in cascade for the amplification of radio signals of either audio or radio frequency, and my invention has equal application both to radio telephony and telegraphy..

In coupling transformers, the amplification of potential varies directly as the ratioof the number of turns in the primary to the number of turns in the secondary. The greatest amplification possible, therefore, takes place when the maximum number of turns are used in the secondary. This means that the number of turns of the secondary of the transformer; i. e., the inductance in the grid-filament circuit of the amplifier tubes, should be the maximum. The maximum practicable number of turns in the grid-filament circuit that forms the secondary of the transformer is that number of turns which brings that circuit into resonance with the average operating frequency.

Heretofore, it has been considered impossible to utilize a resonant grid-filament circuit in the amplifier tubes for the reason that, at the resonant frequency, the tuned grid-filament circuit operates as capacitance conductively coupled in parallel to the inductance of the primary of the transformer; i. e., the inductance in the plate-filament circuit of the tube supplying the oscillations. The effective value of the capacitance was such that the inductance of the primary winding with this effective capacitance was resonant at some frequency. By reason of this phenomenon, the maximum number of turns in the secondary or that number atwhich the secondary is resonant to the .oper-,

ating frequency has never been employed and maximum possible amplification has never been obtained.

My invention comprises the use of a secondary or grid-filament circuit tuned to the average operating frequency. This means that the maximum number of turns will be present in the secondary of the transformer, and hence, the maximum amplification possible will be obtained. To flatten out the impedance-frequency curve for the rimary of the transformer, I employ an indbctance in the plate-filament circuit of large 'value, in proportion t0 the rest of the impedance in said circuit. This gives, for the low fre- I quencies, a large value to the impedance in the primary circuit, hence an increase of the impedance at resonant frequencies does not produce an objectionable change in the ef fective amount of impedance in the primary circuit.

An object of my invention is to get the maximum number of turns in the secondary in intertube transformers for connecting vacuum tubes in the amplification of radio signals.

Another object is to get the maximum number of turns in the secondary of a transformer, in such circuit, without objectionable excessive variations in the impedance of the primary circuit at different fre uencies.

With these and other objects in View, the invention consists in the parts and combina tions to be hereinafter explained, with the understanding that the several necessary elements may be varied in proportion and arrangement without departing from the spirit of theinvention.

In the drawings, Figure 1 illustrates diagrammatically a receiving system in which vacuum tubes, coupled according to my invention, are cascaded to amplify received signals.

Fig. illustrates diagrammatically the equivalent circuit of an intertube transformer for determining the primary impedance.

Fig. 3 is a diagram showing the variation of current values with frequency where inductance and capacity, are connected in parallel.

Fig. 4 is a diagram showing graphically the variation of impedance with frequency in the primary of an intertube transformer.

Referring to the drawing, in Fig. 1 is shown an antenna circuit comprising an antenna 1, an inductance 2 and a variable capacitance 3 through which the circuit is connected to ground; By varying the variable capacitance 3, the circuit may be tuned to any frequency desired. Inductively coupled to the inductance 2 is an inductance 4 in the grid circuit of a three-electrode vacuum tube, 5 operating as an amplifier. Connected in parallel with the inductance 4 is a variable capacitance 6, permitting the circuit to be tuned to any desired frequency. Connected to the inductance 4 is a grid 7 of the electron tube 5. The inductance 4 islikewise con Ice nected to a filament element 10 0f the electron tube 5 which is heated by a sultable battery 11. I

The plate circuit of the vacuum tube 5 comprises a plate element 12, a plate-or l3 battery 13 and an inductance 14. The 1nductance 14 performs the function of the primary of an intertube transformer. The inductance 14 is of relatively high value, in proportion to the rest of the impedance in the plate circuit of the vacuum tube 5 The impedance Z in Fig. 2 shows the equivalent circuit for the primary of the intertube transformer. The impedance Z has a high er impedance for a; broader band of frequencies as the ratio of inductance to capacitance is increased. The capacitance in this primary cir uit is the capacitance of the gird-filament ircuit of a tube and comprises the capacitance between the filament and the grid and the distributed capacitance of the winding.

The'greater the number of turns in the primary, the greater the ratio, therefore, of inductance to capacitance. Since the voltage acting in the plate-filament circuit divides between the two impedances in direct ratio to the value of each, the largerthe value of impedance Z the greater the value of voltage acting upon the primary of the intertube transformer. To have a large voltage acting upon the primary is desirable but there is a point, which can be easily determined, beyond which increase'in the number of turns in the primary is not advantageous. The reason for this lies in the fact that the amplification in the transformer depends uponthe ratio of the number of turns of the primary to the number of turns of the secondary. There is another feature which must be taken into consideration, namely, the proper value of impedance Z, to obtain a suitable flattening-of the impedance curve to enable my system to be used in telephony. A discussion of this question will be later fully dealt with.

The computation of the impedance Z may easilybe made from an inspection of the equivalent circuit shown in Fig. 2. In this figure of the drawings, R represents the internal impedance of the plate circuit and Z the impedance of the transformer primary.-

C is the distributed capacitance of the winding plus the capacitance of the grid-filament circuit of the next vacuum tube in the eascade. The impedance of this primary winding, considered alone, may be represented by the following equation:

where R- is the resistance of the primary 7 represents the frequency: and L, is the inductance of the pr1mary.:

Inductively coupled, through an iron core, to the inductance 14, is an inductance 15 in the grid-filament circuit of an electron tube 16. The grid-filament circuit comprises a grid 17 and a filament 18 heated by means of asuitable battery 19. The grid-filament circuit of the amplifier tube 16 is tuned to resonance at the usual operating frequency greatest possible number of turns are employed in the secondary. This will give the maximum possible amplification.

The platecircuit of the amplifier tube 16 has a plate element 22, a plate battery 23 and an inductance 24, which latter operates as the primary of an intertube transformer. The inductance 24 corresponds, in design and function, exactly to the inductance 14 of the plate circuit of the tube 7. Inductively coupled, through iron, to the inductance 24, forming the primary of the intertube transformer, is an inductance 25, located in the grid-filament circuit of an amplifier tube 26, having a grid element 27 and a filament element 28. The filament 28 is heated by a suitable A battery 29. The grid-filament circuit of the amplifier tube 26 is tuned to be resonant to the operating frequency and, in every respect, corresponds, in function and design, to the grid-filament circuit of the electron tube 16. The plate circuit of the electron tube 26 comprises a plate element 32, a B battery 33 and a pair of telephone receivers 34 through which amplified signals maybe caught.

The connection of the amplifier tubes 5, 16, and 26 is in cascade and produces a greater amplification than is possible with the use of one tube. This is a customary expedient in the art, and my invention does not lie in cascading tubes but in the peculiar use of transformer secondary circuits; i. e., grid filament circuits, tuned to resonance at the customary operating frequency to obtain a maximum amplification.

In the circuit shown in Fig. 2, the dotted lines indicate a capacitance C in parallel with the inductance Z of the primary of the transformer. In this equivalent circuit, the capacitance C represents the equivalent effect of the grid-filament circuit of the amplifier tube. which, with inductance L is resonant at the average operating frequency. This will cause the circuit Z,,C to be a tuned parallel circuit.

The characteristics of a tuned parallel cir cuit will be evident from a study of the curves shown in Fig. 3. In such circuit, the ordinates'are used to plot the current and the abscissa the frequency. It is well known that the current flowing through a reactance is equal to E divided by the value of the reactance. In the case of capacitance, I, therefore, equals EQ-rrfC. In the case of inductance, I equals Since the current through capacitance leads ninety degrees in phase and the current through inductance lags ninety degrees in phase, the resultant current through capacitance and inductance in parallel equals the difference between the values represented by the two curves. This gives us the curve C. At resonant frequency f, the current value is zero for, at that frequency, the capacitance and inductance in parallel theoretically act as infinite resistance. This is true upon the assumption that resistance plays no part 1n the circuit. Since such is never the case, the curve, in'which impedance is the ordinate and frequency the abscissa, is represented by D in Fig. 4, in which the value of the impedance, at the resonant frequency, 1s shown as relatively high though not infinite.

In the past, the trouble with using a secondary resonant to average operating frequency in transformer action betweentubes has been that the impedance in the primary circuit of the transformer changed too much for use in practice, particularly in telephony, around the resonant frequency. But since, as I have before stated, I make my impedance in the primary of relatively high value initially, I obtain a considerable flattening of the impedance curve at resonance. This is illustrated by the dotted line, curve G of i Fig. 4, in which Z represents the impedance at the greatest' distance from resonance. Therefore, the resonant point is sufliciently well obscured to produce an impedance which is constant enough with variable frequency to give satisfactory results in telephony.

Thus, I am able to use a transformer coupling between vacuum tubes in which the secondary; i. e., the grid-filament circuit, contains the necessary number of turns so that it is tuned to resonanceand, therefore, comprises the maximum number of turns. Since the ratio of the number of turns in a secondary of a. transformer to the number of turns in the primary determines the ampli fication, by employing a tuned secondary with the maximum possible number of turns, I can obtain a greater amplification than has yet been achieved. It is necessary to use a winding which has a low distributed capacity, the lower the capacity the greater the number of turns it is possible to use.

In operation, radio oscillations, having been caught by the antenna circuit, induce corresponding oscillations in the inductance 4, by reason of the inductive coupling between the inductance 2'and the inductance 4. By suitable adjustment of the variable capacitance 6, the grid circuit of the vacuum tube 5 may be tuned to any desired frequency. The A battery 11 applies a suitable degree of heat to the filament 10 of the "electron tube to cause it to emit electrons,

ondary 15 of the intertube transformer that lies in the grid-filament circuit of the electron tube 16. The grid-filament. circuit of the electron tube 16 is tuned to resonance with the average operating frequency. This means that the maximum possible number of turns are used in the inductance 15. The inductance 14, forming the primary, is of large enough value, so that the resonant point is sufiiciently well obscured to produce a sufliciently constant impedance in the primary to give satisfactory results in te lephony.

The primary 14 is, however, not of too large value for the reason that the amplification of the intertube transformer depends upon the ratio of the-number of turns in the secondary to the number of turns in the primary. The grid 17 of the electron tube 16 which lies in the grid-filament circuit of that tube has an amplified potential applied to it. This potential, modulated in accordance with the received signals, controls the electron fiow between filament element- 18 and plate element 22 of the electron tube 16. This, in turn, produces amplified oscillations in theplate circuit of the electron tube 16 which comprises the inductance 24. The inductance 24 is similar, in design and function, to the inductance 14 and forms the primary of an intertube transformer the, sec

'ondary of which is the inductance 25. i The inductance 25 lies in the grid-filament circuit of the electron tube 26 and this circuit is tuned to resonance at the average operating frequency.

The grid-filament circuit of the electron tube 26 operates and is designed in exactly the same manner as the grid-filament circuit of the electron tube 16. The tubes 16 and 26 are cascaded for the purpose of furnishing additional amplification. Within the filament-plate circuit of the electron tube 26 are located telephone receivers 34 to catch the amplified signals.

While I have illustrated my system of intert-ube coupling as applied to a receiving system, it is to be distinctly understood that it may be applied to a sending system, and that I have shown a receiving system solely for the purpose of explanation and'to make clear the principles of operation of my invention.

By my form of intcrtube coupling, I am enabled to obtain the maximum voltage amplification possible. The amplification in transformers depends upon the ratio of the number of turns of the primary to the number of turns of the secondary. The greater the number of turns in the secondary the greater will be the amplification of potential. The maximum number of turns practically possible in the secondary is obtained when the distributed capacity of the secondary circuit; i. e., the grid-filament circuit, is tuned to resonance with the inductance of the winding at the average operating frequency. My system has the peculiar advantage that the maximum amplification possible is obtained by reason of the use of a secondary circuit in intertube transformers that shall be resonant to operating frequencies.

Since I have not shown and described all of the possible embodiments of my invention that will be apparent to those skilled in the art, I desire that my invention be limited only by the appended claims and the showing of the prior art.

I claim as my invention:

1. In combination, two vacuum tubes and a transformer, the primary of said transformer being connected to the output of the first tube and the secondary to the input of the second tube, the inductance of said primary being large relative to the output impedance of the first tube and smaller than the inductance of the secondary, the inductance of said secondary being such that, with the distributed capacity thereof and the input capacity of the second tube, it forms a circuit resonant to the average frequency of the currents for which the combination is intended.

2. In an amplifying system, means for supplying currents of freqeuncies comprised within a predetermined frequency band, a pair of vacuum tubes and a transformer between the tubes, the primary of said transformer being connected'in the output circuit of the first of said tubes and having sufficient inductance to constitute it the major part of the impedance of said output circuit both within and without said first tube, the sec ondary of said transformer being connected in the input circuit of the second tube and having suflicient inductance to render said input circuit resonant at the average frequency of said frequency band.

3. In an amplifying system, means for supplying currents of frequencies comprised within a predetermined frequency band, a

pair of vacuum tubes and a closely coupled transformer between the tubes, the primary within a predetermined frequency band, a

pair of vacuum tubes and a closely coupled transformer between the tubes, the primary of said transformer being connected in the output circuit of the first of said tubes and having sufficient inductance to render the transformer of sufficiently broad tuning to transmit effectively variations of the frequency corresponding to modulation and to constitute it the major part of the impedance of said output circuit both within and without said first tube, the secondary of said transformer being connected in the input circuit of the second tube and having sufficient inductance to render said input circuit resonant at the average frequency of said frequency band.

5. In an amplifying system, means for supplying currents of frequencies comprised within a predetermined frequency band, a pair of vacuum tubes and a closely coupled transformer between the tubes, the primary of said transformer being connected in the output circuit of the first of said tubes and having sufficient inductance to so obscure the resonance point of the transformer that it will not produce perceptible distortion and to constitute it the major part ofthe impedance of said output circuit both within and without said first tube, the secondary of said transformer being connected in the input circuit of the second tube and having suflicient inductance to render said input circuit resonant at the average frequency of said frequency band.

6. In combination, two vacuum tubes and a closely coupled transformer, the primary of said transformer being connected to the output of the first tube and the secondary to.

the input of the second tube, the inductance of said secondary being such that, with the distributed capacity thereof and the input capacity of the second tube, it forms a circuit resonant to the average frequency of the currents for which the combination is intended, whereby the sharpness of the tuning of the combination a is determined by the amount of inductance in the primary of said transformer.

In testimony whereof, I have hereunto subscribed my name this 26 day of February,

MAX C. BATSEL.

CERTIFICATE OF GORRECTION.

Patent No. 1,664,239. Granted March 27, 1928, to

MAX. C. BATSEL.

it is hereby certified that the above numbered patent was erroneously issued to the inventor said "Batsei", whereas said patent should have been issued to "Radio Corporation oi eriea", of New York, N. Y., a corporation of Delaware, said corporation being assignee oi the entire interest in said invention, as shown. by the records of assignments in this otiiee; and that thesaid Letters Patent should be read'with this eorreetion therein that the same may conform to the record of the case in the Patent ()itice.

Signed and sealed this 19th day of June, A. D. 1928.

I M. .13. Moore, (Seal) Acting Commissioner of Patents.

CERTIFICATE OF CORRECTION,

Patent No. 1,664,239. Granted March 27, 1928, to

MAX 0. BATSEL.

it is hereby certified that the above numbered patent was erroneously issued to the inventor said "Batsel", whereas said patent should have been issued to "Radio {Iorporation of America", of New York, N. Y., a corporation of Delaware, said corporation being assignee of the entire interest in said invention, as shown by the records of assignments in this office; and that the said Letters Patent shonid be read with this correction therein that the same may conform to the record of the ease in the Patent Qffice.

Signed and sealed this 19th day of June, A. D. 1928.

M. J. Moore, (Seal) Acting Commissioner of Patents. 

